It is well known that ammonia gas can be decomposed by contacting a mixture of ammonia gas and air with a catalyst, for example, made of platinum at an elevated temperature.
The catalysts for use in the decomposition of ammonia are platinum metals such as platinum, rhodium, ruthenium or alloys thereof carried on a round spherical type base of alumina, silica, zeolite, kaolin or silicon carbide having a diameter of 2 mm to 10 mm and having 0.05% to 1.0% by weight of Pt, Rh, Ru or alloy thereof on the base.
The decomposition of ammonia is oxidation thereof and is carried out at a temperature of higher than 180.degree. C. to convert same to nitrogen and water. During the composition of ammonia gas, however, noxious nitrogen oxides (NOx), which are harmful, are ordinarily produced, because NOx are increasingly produced at a temperature of higher than 250.degree. C., especially higher than 300.degree. C., and the decomposition of ammonia is an exothermic reaction accompanied with calorific value of 76.2 kcal/mol, and therefore, as the decomposition proceeds, the temperature of the air containing ammonia gas to be decomposed rises, for example, to a temperature of higher than 300.degree. C. The production of NOx increases progressively as the temperature rises.
The production of NOx can be prevented by maintaining the temperature of the air containing ammonia gas to be decomposed as low as possible in a temperature of higher than 180.degree. C., for example, in a range of from 200.degree. C. to 230.degree. C.
A method of catalytic treatment of gases containing ammonia has been disclosed in U.S. Pat. No. 3,467,491. In this patent, there is shown a method comprising passing a mixture of ammonia gas and air into contact with a platinum-alumina catalyst at a controlled temperature range of from about 400.degree. F. (200.degree. C.) to about 450.degree. F. (230.degree. C.) to effect a conversion to nitrogen and water without production of noxious nitrogen oxides (NOx). In the above U.S. patent, there is not provided a method controlling the temperature of the ammonia-containing air on decomposition. It is an object of the present invention to provide a catalyst bed in which the temperature of the ammonia-containing air to be subjected to catalytic action is controlled to minimize the amount of noxious nitrogen oxides produced on decomposition of ammonia gas to nitrogen and water.